This disclosure relates to magnetorheological fluid compositions, and more particularly to high yield stress magnetorheological (MR) fluid compositions.
Fluid compositions that undergo a change in apparent viscosity in the presence of a magnetic field are referred to as Bingham magnetic fluids or magnetorheological fluids. Magnetorheological fluids generally include low aspect ratio magnetizable particles dispersed or suspended in a carrier fluid. The low aspect ratio magnetizable particles have an aspect ratio less than 1.5, and more typically have an aspect ratio of about 1. In the presence of a magnetic field, the low aspect magnetizable particles become polarized and are thereby organized into chains of particles within the carrier fluid. The chains of particles act to increase the apparent viscosity or flow resistance of the fluid composition resulting in the development of a solid mass having a yield stress that must be exceeded to induce onset of flow of the magnetorheological fluid. When the flow of the fluid composition is restricted as a result of orientation of the particles into chains, the fluid composition is said to be in its “on state”. The force required to exceed the yield stress is referred to as the “yield strength”. In the absence of a magnetic field, the particles return to a disorganized or free state and the apparent viscosity or flow resistance of the fluid composition is then correspondingly reduced. The state occupied by the composition in the absence of a magnetic field is referred to as the “off-state”.
The carrier fluids employed in the MR fluid composition form the continuous phase in which the magnetic particles are dispersed or suspended. Prior art carrier fluids are generally organic. Specific examples of prior art carrier fluids are natural fatty oils, mineral oils, poly α-olefins, polyphenylethers, polyesters (such as perfluorinated polyesters, dibasic acid esters and neopentylpolyol esters), phosphate esters, synthetic cycloparaffin oils and synthetic paraffin oils, unsaturated hydrocarbon oils, monobasic acid esters, glycol esters and ethers (such as polyalkylene glycol), synthetic hydrocarbon oils, perfluorinated polyethers, halogenated hydrocarbons, or the like, or a combination comprising at least one of the foregoing carrier fluids. Because of the relatively low specific gravity of these carrier fluids, the MR fluid compositions typically include a suspending agent such as fumed silica, clay, nanoparticles or the like. Optionally, particle settling in these types of carrier fluids can be managed through the use of other additives or treatments, which allow for re-suspension of the particles.
The prior art carrier fluids are generally unsuitable for high temperature and high yield stress applications, wherein the operating temperatures of the device using the MR fluid composition exceed 100° C. or more. At these temperatures, current MR fluid compositions can deteriorate causing changes in performance during operation of the device. For example, a change in yield stress in the on-state or an increase in viscosity in the off-state, among others, typically occurs. The amount of deterioration generally depends on shear rate, temperature, and duration. In addition, because these fluids generally are of low specific gravity, the compositions can exhibit unacceptable particle settling. As such, current MR fluid compositions are generally unsuitable for such high temperature applications as a clutch application for a vehicle alternator, which can result in the MR fluid composition being exposed to temperatures of about 200 to about 250° C. (with transients at about 450° C. or more); a transmission clutch that generally operates at a temperature of 100° C. or more; a variable valve actuator disposed in the exhaust stems near the cylinder head, wherein the MR fluid composition can be exposed to operating temperatures of about 400-500° C.; and the like.
Desirable MR fluid properties for the aforementioned high temperature applications include, among others, a low viscosity, a high temperature capability, and a low tendency for particle settling. It is difficult to achieve most, if not all, of these properties with the prior art carrier fluids. For example, silicone fluids offer better heat resistance relative to other types of prior art carrier fluids but have never been found to work satisfactorily in high temperature applications requiring rapid (on the order of milliseconds) and reversible changes in yield stress such as the clutch applications described above. Moreover, silicone fluids, operating at high temperature conditions, are prone to crosslinking, which directly affects the off-state properties and operating lifetimes.
Accordingly, there is a need for improved high temperature MR fluid compositions that can meet the needs of devices used for high temperature applications.